When performing nondestructive inspection of a fine internal structure of an inspection target by fluoroscopic X-rays or X-ray CT, for the purpose of obtaining a clear X-ray image with no blurring, a micro-focus X-ray inspection device using an X-ray tube having a micro-focus is used. An X-ray tube used for a micro-focus X-ray inspection device or the like realizes a small X-ray focal point by irradiating a target with an electron beam narrowed down to a μm level by a magnetic lens (see Patent Document 1).
One example of a configuration of an X-ray tube used for such a micro-focus X-ray inspection device is shown in FIG. 3. From a filament (electron source) 12 serving as a negative electrode to which a negative voltage is applied in a high-vacuum vacuum chamber 11 to which a vacuum gauge G and a turbo molecular pump TMP are attached, an electron beam B is emitted toward the grounded anode 13. At the center of the anode 13, a hole 13a is provided. The electron beam B is accelerated to pass through the hole 13a of the anode 13 and further pass through a cylindrical holder shaft 14 communicating with the hole 13a, and is irradiated onto the target 16 arranged in a target holder 15. The outside of the target holder 15 is cooled by a water cooling mechanism 15a (which may be an air cooling mechanism).
On the outside of the holder shaft 14, a magnetic lens 17 for converging the electron beam B and a deflector 18 for adjusting the direction of the electron beam B are provided. The electron beam B passing through the holder shaft 14 is narrowed down to the μm level by the magnetic lens 17 and is focused on the X-ray focal point on the target 16.
The target 16 is provided on the tip end side of the magnetic lens 17. In order to reduce the focal point to a small value, it is necessary to make the tip end portion of the magnetic lens 17 completely axially symmetrical. Since the symmetry is lost when a fixing portion, such as, e.g., a fixing hole, is provided in the magnetic lens 17, the holder shaft 14 is airtightly fixed by the O-ring seal 20 via the flange 19 on the anode 13 side.
The holder shaft 14 through which the electron beam B passes has an inner diameter of about 10 mm. This holder shaft 14 is required to be less likely to be magnetized, have heat dissipation properties, and have a high melting point since the inner wall locally reaches a high temperature when the electron beam B hits the inner wall. A tungsten alloy is used as a material meeting these requirements.
That is, tungsten is a nonmagnetic heavy metal having a melting point of 3,685 K and has sufficient resistance to a local temperature rise due to an electron beam. However, tungsten as a single metal is inferior in workability, and therefore it is used as a tungsten alloy to give ease of processing. For the target holder 15 as well, a tungsten alloy is used from the viewpoint of preventing the emission of X-rays from directions other than the X-ray irradiation window, and the target holder 15 and the holder shaft 14 are fixed by brazing.